1. Field of the Invention
This invention relates to electrical devices comprising PTC elements.
2. Summary of the Prior Art
Conductive polymer compositions, i.e. compositions comprising a polymer and conductive particles dispersed in the polymer, are well known. Over recent years there has been particular interest in such compositions which exhibit positive temperature coefficient (PTC) behavior, i.e. which show a sharp increase in resistivity over a particular range, and in electrical devices comprising PTC elements composed of such PTC compositions. Reference may be made for example to U.S. Pat. No. 3,858,144 and to copending and commonly assigned Application Ser. Nos. 601,638 (Horsma et al.), U.S. Pat. No. 4,177,376, 750,149 (Kamath et al.), now abandoned, 751,095 (Toy et al.), now abandoned, 798,154 (Horsma), now abandoned 873,676 (Horsma), 965,343 (Van Konynenburg et al.) and 965,344 (Middleman et al.) and the continuation-in-part of Ser. No. 965,344 Ser. No. 98,712 filed contemporaneously with this application. The disclosure of this patent and these applications is incorporated by reference herein. It is known that devices of this kind may comprise a jacket of a polymeric material which insulates the device electrically and also provides physical protection. Thus the self-limiting PTC heaters have insulating jackets of thermoplastic polymers which may be cross-linked. U.S. Pat. No. 3,914,363 (Bedard) discloses that it is useful for the jacket to have residual stress at temperatures used for annealing the PTC composition to reduce its resistivity. U.S. Pat. No. 3,351,882 (Kohler et al.) discloses that a PTC device may have a casing of any suitable known epoxy resin or silicone rubber, but does not give any specific example of such a device.
In an article in Journal of Polymer Engineering and Science, 14, 706 (1974), J. Meyer discloses that the presence of an anti-oxidant, e.g. a hindered phenol, in a PTC composition influences the way in which the electrical properties of the composition change when the device is subjected to aging at elevated temperature.